Of or related to the styles, community, culture, or behaviours of Mongkok, a Kowloon neighbourhood in Hong Kong known for the alternative crowds it attracts such as punk, hipster, goth, and other non-traditional subcultures. The neighbourhood is often compared to Harajuku in Tokyo or even a Williamsburg, Brooklyn, NYC.

"Do you really wanna buy those black boots? You'll look so MK if you wear then with tight ted jeans."

Standing outside the MTR station while smoking and playing with your handset is so MK"

Used colloquially in British Columbia, to describe the action of selling out without good reason. The term is thought to originate from a metro vancouverite named Mark Kim, well known to his community as a martial artist and educated person. This potential origin is backed up by evidence of Mark Kim calling off his attendance to several events for poor excuses such as fatigue or lack of interest.

Mk is the slang used for saying Okay or "you okay" in a question. So say you asked someone to pick up onions, you would say "Pick up some onions, mk?"
You pronounce and say it the way that the M is not pronounced like the alphabet, but the way it would be pronounced in a word, for example the M at the beginning of Megan. The K is pronounced the alphabet way, however, i.e. not in the way you would say "kite". It also can be used to ask someone to do something, as in "Pick up Tommy, mk?" Instead of "Can you pick up Tommy?"